Flash memory is a commonly used type of non-volatile memory in widespread use as storage for consumer electronics and mass storage applications. Flash memory is pervasive in popular consumer products such as digital audio/video players, cell phones and digital cameras, for storing application data and/or media data. Flash memory can further be used as a dedicated storage device, such as a portable flash drive pluggable into a universal serial port (USB) of a personal computer, and a magnetic hard disk drive (HDD) replacement for example. It is well known that flash memory is non-volatile, meaning that it retains stored data in the absence of power, which provides a power savings advantage for the above mentioned consumer products. Flash memory is suited for such applications due to its relatively high density for a given area of its memory array.
Flash memory is organized into blocks, which are erasable. Known flash memory devices suffer inefficiencies in cost, performance and power resulting from their erase schemes. Erasing can be power inefficient as large wells within which the memory cells of the memory blocks are fabricated, are repeatedly charged and discharged during erase and erase verify operations. Accordingly, large charge pumps are needed on-chip, which occupy chip area and thereby increases chip size and cost. Furthermore, the exposure of unselected memory blocks to erase voltages used for erasing selected memory blocks can cause erase disturb, which could inadvertently change the programmed threshold voltages of the memory cells of the unselected memory blocks. This problem is exacerbated in multi-level flash cells which have tighter threshold voltage distributions and are therefore more sensitive to the effects of erase disturb.